Offset on a '73 Camaro
I have a 78 t/a and I'll be putting 17x11's with 5.75" backspacing and 315/35/17 tires in the back. Up front I'm going to have 17x9.5 with 5.5" backspacing and 275/40/17's. I haven't done this yet so I can't say for absolute certainty it will work, but based on what I've been told it should.
And after much agonizing, I've just about convinced myself to go with the wheels the guy has on the following photo. They just look too damn good despite the huge price tag. When asked what size and backspacing he has, he said: "9" x 18" front and 10" x 18" rear. Backspacings are approx 5" front and 6" rear. Tires are BFG KDW 285/40 and 255/45."
HOWEVER - the t/a's have slightly more clearance than the camaros. Based on what I've seen on the pro-touring site, the 18x9's will likely work for you with 5.5" backspacing. But I highly doubt that's the backspacing that comes on C5's. I'm sure someone else can answer that for you.
Anyway, this is about what my car is going to look like when I'm done:


